Modernize setuptools usage in pyproject.toml

1. Remove the redundant `wheel` dependency.  The setuptools build
   backend has been adding it automatically since day one, and it was
   explicitly specified in the docs as a mistake.  See:
   https://github.com/pypa/setuptools/commit/f7d30a9529378cf69054b5176249e5457aaf640a

2. Replace the legacy backend with the regular backend.  The legacy
   backend was only intended to be used implicitly when `pyproject.toml`
   does not specify only, and was not supposed to be specified
   explicitly there.  See:
   https://github.com/pypa/setuptools/issues/1689

3. Prepend the current directory to `sys.path` as required for
   `setup.py` to reliably import `setupbase`.  The non-legacy backend
   no longer does this for us.

pyproject.toml

@@ -1,3 +1,3 b''
 [build-system]
-requires = ["setuptools >= 51.0.0", "wheel"]
-build-backend = "setuptools.build_meta:__legacy__"
+requires = ["setuptools >= 51.0.0"]
+build-backend = "setuptools.build_meta"

setup.py

@@ -1,146 +1,148 b''
 # -*- coding: utf-8 -*-
 """Setup script for IPython.
 
 Under Posix environments it works like a typical setup.py script.
 Under Windows, the command sdist is not supported, since IPython
 requires utilities which are not available under Windows."""
 
 #-----------------------------------------------------------------------------
 #  Copyright (c) 2008-2011, IPython Development Team.
 #  Copyright (c) 2001-2007, Fernando Perez <[e-mail]>
 #  Copyright (c) 2001, Janko Hauser <[e-mail]>
 #  Copyright (c) 2001, Nathaniel Gray <[e-mail]>
 #
 #  Distributed under the terms of the Modified BSD License.
 #
 #  The full license is in the file COPYING.rst, distributed with this software.
 #-----------------------------------------------------------------------------
 
 import os
 import sys
 
 # **Python version check**
 #
 # This check is also made in IPython/__init__, don't forget to update both when
 # changing Python version requirements.
 if sys.version_info < (3, 8):
     pip_message = 'This may be due to an out of date pip. Make sure you have pip >= 9.0.1.'
     try:
         import pip
         pip_version = tuple([int(x) for x in pip.__version__.split('.')[:3]])
         if pip_version < (9, 0, 1) :
             pip_message = 'Your pip version is out of date, please install pip >= 9.0.1. '\
             'pip {} detected.'.format(pip.__version__)
         else:
             # pip is new enough - it must be something else
             pip_message = ''
     except Exception:
         pass
 
 
     error = """
 IPython 8+ supports Python 3.8 and above, following NEP 29.
 When using Python 2.7, please install IPython 5.x LTS Long Term Support version.
 Python 3.3 and 3.4 were supported up to IPython 6.x.
 Python 3.5 was supported with IPython 7.0 to 7.9.
 Python 3.6 was supported with IPython up to 7.16.
 Python 3.7 was still supported with the 7.x branch.
 
 See IPython `README.rst` file for more information:
 
     https://github.com/ipython/ipython/blob/master/README.rst
 
 Python {py} detected.
 {pip}
 """.format(
         py=sys.version_info, pip=pip_message
     )
 
     print(error, file=sys.stderr)
     sys.exit(1)
 
 # At least we're on the python version we need, move on.
 
 from setuptools import setup
 
 # Our own imports
+sys.path.insert(0, ".")
+
 from setupbase import target_update
 
 from setupbase import (
     setup_args,
     check_package_data_first,
     find_data_files,
     git_prebuild,
     install_symlinked,
     install_lib_symlink,
     install_scripts_for_symlink,
     unsymlink,
 )
 
 #-------------------------------------------------------------------------------
 # Handle OS specific things
 #-------------------------------------------------------------------------------
 
 if os.name in ('nt','dos'):
     os_name = 'windows'
 else:
     os_name = os.name
 
 # Under Windows, 'sdist' has not been supported.  Now that the docs build with
 # Sphinx it might work, but let's not turn it on until someone confirms that it
 # actually works.
 if os_name == 'windows' and 'sdist' in sys.argv:
     print('The sdist command is not available under Windows.  Exiting.')
     sys.exit(1)
 
 
 #-------------------------------------------------------------------------------
 # Things related to the IPython documentation
 #-------------------------------------------------------------------------------
 
 # update the manuals when building a source dist
 if len(sys.argv) >= 2 and sys.argv[1] in ('sdist','bdist_rpm'):
 
     # List of things to be updated. Each entry is a triplet of args for
     # target_update()
     to_update = [
         (
             "docs/man/ipython.1.gz",
             ["docs/man/ipython.1"],
             "cd docs/man && python -m gzip --best ipython.1",
         ),
     ]
 
 
     [ target_update(*t) for t in to_update ]
 
 #---------------------------------------------------------------------------
 # Find all the packages, package data, and data_files
 #---------------------------------------------------------------------------
 
 data_files = find_data_files()
 
 setup_args['data_files'] = data_files
 
 #---------------------------------------------------------------------------
 # custom distutils commands
 #---------------------------------------------------------------------------
 # imports here, so they are after setuptools import if there was one
 from setuptools.command.sdist import sdist
 
 setup_args['cmdclass'] = {
     'build_py': \
             check_package_data_first(git_prebuild('IPython')),
     'sdist' : git_prebuild('IPython', sdist),
     'symlink': install_symlinked,
     'install_lib_symlink': install_lib_symlink,
     'install_scripts_sym': install_scripts_for_symlink,
     'unsymlink': unsymlink,
 }
 
 #---------------------------------------------------------------------------
 # Do the actual setup now
 #---------------------------------------------------------------------------
 
 if __name__ == "__main__":
     setup(**setup_args)